better names and a comment

compiler/rustc_hir_analysis/src/check/compare_impl_item.rs

@@ -441,8 +441,41 @@ fn compare_asyncness<'tcx>(
     Ok(())
 }
 
+/// Given a method def-id in an impl, compare the method signature of the impl
+/// against the trait that it's implementing. In doing so, infer the hidden types
+/// that this method's signature provides to satisfy each return-position `impl Trait`
+/// in the trait signature.
+///
+/// The method is also responsible for making sure that the hidden types for each
+/// RPITIT actually satisfy the bounds of the `impl Trait`, i.e. that if we infer
+/// `impl Trait = Foo`, that `Foo: Trait` holds.
+///
+/// For example, given the sample code:
+///
+/// ```
+/// #![feature(return_position_impl_trait_in_trait)]
+///
+/// use std::ops::Deref;
+///
+/// trait Foo {
+///     fn bar() -> impl Deref<Target = impl Sized>;
+///              // ^- RPITIT #1        ^- RPITIT #2
+/// }
+///
+/// impl Foo for () {
+///     fn bar() -> Box<String> { Box::new(String::new()) }
+/// }
+/// ```
+///
+/// The hidden types for the RPITITs in `bar` would be inferred to:
+///     * `impl Deref` (RPITIT #1) = `Box<String>`
+///     * `impl Sized` (RPITIT #2) = `String`
+///
+/// The relationship between these two types is straightforward in this case, but
+/// may be more tenuously connected via other `impl`s and normalization rules for
+/// cases of more complicated nested RPITITs.
 #[instrument(skip(tcx), level = "debug", ret)]
-pub(super) fn collect_trait_impl_trait_tys<'tcx>(
+pub(super) fn collect_return_position_impl_trait_in_trait_tys<'tcx>(
     tcx: TyCtxt<'tcx>,
     def_id: DefId,
 ) -> Result<&'tcx FxHashMap<DefId, Ty<'tcx>>, ErrorGuaranteed> {

compiler/rustc_hir_analysis/src/check/mod.rs

@@ -94,7 +94,7 @@
 use crate::require_c_abi_if_c_variadic;
 use crate::util::common::indenter;
 
-use self::compare_impl_item::collect_trait_impl_trait_tys;
+use self::compare_impl_item::collect_return_position_impl_trait_in_trait_tys;
 use self::region::region_scope_tree;
 
 pub fn provide(providers: &mut Providers) {
@@ -103,7 +103,7 @@ pub fn provide(providers: &mut Providers) {
         adt_destructor,
         check_mod_item_types,
         region_scope_tree,
-        collect_trait_impl_trait_tys,
+        collect_return_position_impl_trait_in_trait_tys,
         compare_impl_const: compare_impl_item::compare_impl_const_raw,
         ..*providers
     };

compiler/rustc_metadata/src/rmeta/decoder/cstore_impl.rs

@@ -223,7 +223,7 @@ fn into_args(self) -> (DefId, SimplifiedType) {
     generator_kind => { table }
     trait_def => { table }
     deduced_param_attrs => { table }
-    collect_trait_impl_trait_tys => {
+    collect_return_position_impl_trait_in_trait_tys => {
         Ok(cdata
             .root
             .tables

compiler/rustc_metadata/src/rmeta/encoder.rs

@@ -1197,7 +1197,7 @@ fn encode_def_ids(&mut self) {
                 record!(self.tables.params_in_repr[def_id] <- params_in_repr);
             }
             if should_encode_trait_impl_trait_tys(tcx, def_id)
-                && let Ok(table) = self.tcx.collect_trait_impl_trait_tys(def_id)
+                && let Ok(table) = self.tcx.collect_return_position_impl_trait_in_trait_tys(def_id)
             {
                 record!(self.tables.trait_impl_trait_tys[def_id] <- table);
             }

compiler/rustc_middle/src/query/mod.rs

@@ -169,7 +169,7 @@
         separate_provide_extern
     }
 
-    query collect_trait_impl_trait_tys(key: DefId)
+    query collect_return_position_impl_trait_in_trait_tys(key: DefId)
         -> Result<&'tcx FxHashMap<DefId, Ty<'tcx>>, ErrorGuaranteed>
     {
         desc { "comparing an impl and trait method signature, inferring any hidden `impl Trait` types in the process" }

compiler/rustc_middle/src/ty/util.rs

@@ -641,11 +641,11 @@ pub fn bound_type_of(self, def_id: DefId) -> ty::EarlyBinder<Ty<'tcx>> {
         ty::EarlyBinder(self.type_of(def_id))
     }
 
-    pub fn bound_trait_impl_trait_tys(
+    pub fn bound_return_position_impl_trait_in_trait_tys(
         self,
         def_id: DefId,
     ) -> ty::EarlyBinder<Result<&'tcx FxHashMap<DefId, Ty<'tcx>>, ErrorGuaranteed>> {
-        ty::EarlyBinder(self.collect_trait_impl_trait_tys(def_id))
+        ty::EarlyBinder(self.collect_return_position_impl_trait_in_trait_tys(def_id))
     }
 
     pub fn bound_fn_sig(self, def_id: DefId) -> ty::EarlyBinder<ty::PolyFnSig<'tcx>> {

compiler/rustc_trait_selection/src/traits/project.rs

@@ -2284,7 +2284,7 @@ fn confirm_impl_trait_in_trait_candidate<'tcx>(
         obligation.param_env,
         cause.clone(),
         obligation.recursion_depth + 1,
-        tcx.bound_trait_impl_trait_tys(impl_fn_def_id)
+        tcx.bound_return_position_impl_trait_in_trait_tys(impl_fn_def_id)
             .map_bound(|tys| {
                 tys.map_or_else(|_| tcx.ty_error(), |tys| tys[&obligation.predicate.def_id])
             })